Just how ineffective is this year's flu shot? CDC experts do the math

Ryan Hernandez grimaces while getting a flu shot in Victorville, Calif., in November. A new study says the vaccine reduced the risk of serious illness by 23%. (James Quigg / The Victor Valley Daily Press/Associated Press)

If you think this year’s flu shot is worthless, think again: People who got vaccinated this fall or winter have been 23% less likely than their unvaccinated peers to come down with a flu-like illness bad enough to send them to the doctor, according to a new report from researchers at the Centers for Disease Control and Prevention.

That 23% figure is a measure known as “vaccine effectiveness,” and it’s certainly on the low end of the spectrum. In the decade since experts began calculating a “VE” for flu vaccines, it has ranged from a low of 10% to a high of 60%.

But even at 10%, a vaccine could prevent about 13,000 flu-related hospitalizations among senior citizens in the U.S., the CDC researchers and their colleagues wrote in Thursday's edition of the Morbidity and Mortality Weekly Report.

The research team arrived at the 23% figure by interviewing patients whose doctors were participating in the U.S. Flu VE Network, a surveillance system with sites in Michigan, Pennsylvania, Texas, Wisconsin and Washington state. Patients were eligible for the study if they were at least 6 months old (and thus eligible to get a flu vaccine), had an acute respiratory illness with a cough, and had not yet taken any flu medicines, like Tamiflu or Relenza.

If patients agreed to participate, researchers used a genetic test to see whether they indeed had the flu (and if so, which strain). They also asked volunteers whether they had gotten a flu shot or flu mist vaccine at least two weeks before they became sick.

As of Jan. 2, 2,321 children and adults had joined the study. The researchers found that 49% of the people who tested positive for the flu had been vaccinated, along with a slightly higher percentage – 56% – of those who tested negative.

After the researchers controlled for factors like age, gender and baseline health, they determined that the vaccine reduced the risk of needing to see a doctor for treatment of a flu-like illness by 23% overall.

But the vaccine didn’t help everyone equally. Kids benefited the most – the VE for those between the ages of 6 months and 17 years was 26%. Among adults, the VE was 12% for people ages 18 to 49 and 14% for people 50 and older. The figures for adults were too small to be statistically significant.

Normally, researchers don’t make calculations about vaccine effectiveness until later in the flu season. But since flu activity is already widespread in 46 states (including all five states in the Flu VE Network), the researchers had enough data to make a first pass at their calculations. As the flu season wears on and more volunteers join the study, the VE may rise (or, in the case of adults, the lower VEs may become statistically significant).

Dr. Tom Frieden, the director of the CDC, has warned that this year's flu season could cause more sickness and death than usual because H3N2 flu strains are dominating. Making matters worse, the North American vaccine is a good match for only about one-third of the H3N2 viruses that are circulating here. The rest are of a type that didn't show up here until March – after the World Health Organization had decided which strains the vaccine should target.

In years when the vaccine is a good match for circulating viruses, the vaccine effectiveness has been in the 50% to 60% range.

Despite the limitations of this year’s vaccine, the study authors still strongly recommended that people who haven’t gotten it yet should do so. It “can still prevent some infections with the circulating (A) H3N2 viruses and might also prevent serious complications requiring hospitalization,” they wrote. “Also, vaccine might protect against other influenza viruses that can circulate later.” The flu shot and flu mist have been well matched to the H1N1 and other flu viruses that have turned up in lab tests, according to the CDC’s FluView reports.